Compressed air supply units are used in vehicles of all kinds, especially for supplying air suspension units of passenger cars with compressed air. Air suspension units can also comprise level control devices, by means of which the clearance between the vehicle axle and the vehicle body can be adjusted. An air suspension system of a pneumatic compressed air supply system comprises a number of air bellows that are pneumatically connected to a common line (gallery), which can raise the vehicle body as they are filled to an increasing extent and can lower it when they are filled to a decreasing extent. Such a system has application in all-terrain vehicles (ATVs) and sports utility vehicles (SUVs). A compressed air supply unit for use in a pneumatic compressed air supply system having a pneumatic unit, e.g., having an air suspension unit as described above, is operated with compressed air from a compressed air feed, e.g., within the range of a pressure level of 5 to 20 bar. The compressed air is made available to the compressed air feed by means of a compressor. On one hand, the compressed air feed is pneumatically linked to a compressed air connection in order to supply the pneumatic unit; on the other hand, the compressed air feed is pneumatically linked to a vent connection. By means of a vent valve arrangement, the compressed air supply unit and/or the pneumatic unit can be vented by releasing air toward the vent connection.
To ensure long-term operation of the compressor supply unit, the unit has an air dryer, by means of which the compressed air can be dried. Accumulation of moisture in the compressed air supply unit is thereby avoided, which can otherwise lead to valve-damaging crystal formation and other unwanted effects in the compressed air supply unit and in the pneumatic unit at relatively low temperatures. An air dryer has a drying agent, generally loose granules, through which the compressed air can flow, allowing the loose granules to take up moisture contained in the compressed air by adsorption at a relatively high pressure. One proven practice is to accommodate the drying granules in a separate replaceable dryer cartridge, which is inserted into a dryer housing. A dryer cartridge of this kind has a dryer bed for the U-shaped or alternately opposed routing of a compressed air flow in an arrangement of annular spaces through which there is to be flow. The dryer cartridge can be replaced easily. On the other hand, the routing of the compressed air flow in the annular spaces through which there is to be flow lengthens the dryer bed and thus improves the drying of the compressed air.
A dryer cartridge for a commercial vehicle, which is known from WO 2005/051521 A1, for example, typically has an outer annular space and an inner central space. A compressed air flow flows through the entire outer annular space of the dryer cartridge, is diverted to the central space in a dome above the annular space, and is routed in a countercurrent direction parallel to the outer annular space in the central space; the compressed air flow leaves the dryer cartridge on the same side as it flows in. DE 10 2006 037 307 A1 discloses a dryer cartridge, which provides meandering nesting of such annular spaces, wherein, following the same principle, there is parallel and countercurrent flow through the annular spaces by a compressed air flow; i.e., a drying container in the form of the dryer cartridge has an outer container outer wall and a number of annular walls along a longitudinal extent of the dryer cartridge, wherein the annular walls divide a dryer cartridge interior delimited by the container outer wall into a large number of annular spaces that are pneumatically linked and allow parallel and countercurrent through-flow.
An alternative embodiment of a replaceable dryer cartridge is known from EP 1 048 540 A1, wherein the dryer cartridge is formed with a container outer wall, the internal space of which is subdivided by a number of loosely inserted separate dividing plates extending over the cross section of the internal space into partitions that extend over the entire cross section of the internal space. The compressed air flow enters the dryer cartridge via a narrow outer annular space, is diverted in a dome thereof, and flows transversely through the partitions between the plates to a longitudinal extent of the dryer cartridge. A dryer cartridge of this kind causes a relatively high pressure loss in a compressed air flow.
EP 1 233 183 B1 describes a compressed air control device having an air dryer. A pot-shaped drying container is inserted in the air dryer housing. The interior of the container can be linked via the housing to a compressed air source, on the one hand, and to a pressure accumulator, on the other hand, by means of a valve arrangement. The valve arrangement provides a first, second and third controllable directional valve, wherein two of the valves are used to open a first and a second passage in the bottom of the drying container. Opposite the bottom of the drying container, the container is open, wherein the granules contained in the drying container are held by an end plate, which is centered on a central bar and secured thereon at the end by means of a spring clip. There can be flow in two directions, for emptying and filling the pressure accumulator, through the container internal space surrounding the bar. This construction is relatively complex and its functioning can be improved.
Improved air drying, in particular with an improved dryer bed, combined with a pressure loss that is still acceptable is desirable. Also, the improved air drying should be capable of being implemented in a space-saving manner, and should be suitable for use in a passenger car.